It's because the ceremonial precepts of the Old Law were no longer in force at the coming of Christ. The ceremonial precepts were only intended to help prepare the Israelites for His coming. As St. Thomas Aquinas writes addressing the question "Whether the ceremonies of the Old Law ceased at the coming of Christ?":

All the ceremonial precepts of the Old Law were ordained to the worship of God as stated above (Question [101], Articles [1],2). Now external worship should be in proportion to the internal worship, which consists in faith, hope and charity. Consequently exterior worship had to be subject to variations according to the variations in the internal worship, in which a threefold state may be distinguished. One state was in respect of faith and hope, both in heavenly goods, and in the means of obtaining them—in both of these considered as things to come. Such was the state of faith and hope in the Old Law. Another state of interior worship is that in which we have faith and hope in heavenly goods as things to come; but in the means of obtaining heavenly goods, as in things present or past. Such is the state of the New Law. The third state is that in which both are possessed as present; wherein nothing is believed in as lacking, nothing hoped for as being yet to come. Such is the state of the Blessed [in heaven].

In this state of the Blessed, then, nothing in regard to worship of God will be figurative; there will be naught but "thanksgiving and voice of praise" (Is. 51:3). Hence it is written concerning the city of the Blessed (Apoc. 21:22): "I saw no temple therein: for the Lord God Almighty is the temple thereof, and the Lamb." Proportionately, therefore, the ceremonies of the first-mentioned state which foreshadowed the second and third states, had need to cease at the advent of the second state; and other ceremonies had to be introduced which would be in keeping with the state of divine worship for that particular time, wherein heavenly goods are a thing of the future, but the Divine favors whereby we obtain the heavenly boons are a thing of the present.

Cornelius à Lapide, S.J., commentates on Mt. 15:2-3:

Why do thy disciples, &c. Bread, in this verse, is a common Hebraism for all kinds oi food. Observe : Moses, m the old Law,enjoined, by God's command, that the Jews should abstain from contact with the dead bodies of rapacious birds and unclean animals,from lepers and various other persons and things. And if any one touched them accidentally, he was accounted unclean ; and was not allowed to enter the Temple until he had performed the ablutions which the law prescribed. And these corporeal washings were enjoined for the purpose of adumbrating to the dense minds of the Jews those spiritual washings of the soul, which are effected by contrition and repentance. (See what I have said on Levit. xi. 31.)

The Jews then, but especially the Pharisees, who wished to be accounted more religious than other people, made their whole sanctity consist in such outward and frequent washings before their meals,yea even when they were taking their food, as seems to be her eintimated. This was why, at the wedding-feast at Cana, there were placed six water-pots for these purificatory purposes. This was why they so frequently washed the cups and basons, out of which they ate and drank, yea even their beds and tables, as may be seen in Mark vii. 4. They were thus careful, lest if the vessels out of which they ate were polluted, they should contaminate those who ate out of them. But all this was merely done out of custom, since the law prescribed nothing of the kind.

Observe 2. This excessive scrupulosity of the Jews was little if indeed at all conducive to piety, or profit, since it kept them wholly intent upon external washings. And so it called off their minds from the interior care and purification of the mind from sin. Neither did God require of them this exceeding anxiety about external lustrations; but seems rather to have discouraged it. (Deut. iv. 2) Christ therefore being about to put an end to these vain and frivolous, or noxious traditions, and being desirous of directing their whole attention to the purification of the mind, was unwilling to observe these ablutions, or to enjoin them upon His disciples, although He did not say so in express words, in order to avoid the envy and calumnies of the Pharisees.

[…]

Jesus answered, &c., for the sake of your tradition. Arabic, for the sake of your ordinances. Instead of, for the sake of, some translate, through. But the meaning is the same in both; viz., your traditions set at nought and violate the Law of God. Therefore they are false and impious, and ought not to be observed.

Note the word, your. Your traditions were not instituted by God,or His Saints : nor by the ancient Patriarchs and Prophets. They were only invented in recent times by the Scribes and Pharisees,your predecessors. And you wish to maintain them, not from love and reverence for them, but because ye have come into their place,and because ye would arrogate to yourselves power and authority to ordain similar traditions. But there are Divine and Patriarchal traditions, which must be in every way observed. They are, that the Books of Moses, Isaiah, Jeremiah, Daniel, and the rest of the Prophets are Canonical : that God is One in Essence, Three in Person: that sins are blotted out by true contrition arising from the love of God :that infants are guilty of original sin, and therefore must be cleansed by the Sacrament which God has ordained, and so on. These traditions ye ignore, or make of none effect, O ye Scribes ; being wholly taken up with your own traditions.

^{All the ceremonial precepts of the Old Law were ordained to the worship of God as stated above (Question [101], Articles [1],2). Now external worship should be in proportion to the internal worship, which consists in faith, hope and charity. Consequently exterior worship had to be subject to variations according to the variations in the internal worship, in which a threefold state may be distinguished. One state was in respect of faith and hope, both in heavenly goods, and in the means of obtaining them—in both of these considered as things to come. Such was the state of faith and hope in the Old Law. Another state of interior worship is that in which we have faith and hope in heavenly goods as things to come; but in the means of obtaining heavenly goods, as in things present or past. Such is the state of the New Law. The third state is that in which both are possessed as present; wherein nothing is believed in as lacking, nothing hoped for as being yet to come. Such is the state of the Blessed [in heaven].}

^{In this state of the Blessed, then, nothing in regard to worship of God will be figurative; there will be naught but "thanksgiving and voice of praise" (Is. 51:3). Hence it is written concerning the city of the Blessed (Apoc. 21:22): "I saw no temple therein: for the Lord God Almighty is the temple thereof, and the Lamb." Proportionately, therefore, the ceremonies of the first-mentioned state which foreshadowed the second and third states, had need to cease at the advent of the second state; and other ceremonies had to be introduced which would be in keeping with the state of divine worship for that particular time, wherein heavenly goods are a thing of the future, but the Divine favors whereby we obtain the heavenly boons are a thing of the present.}